The well known and widely used Internet Group Management Protocol (IGMP) is used by IP hosts to report their multicast group memberships to any immediately neighboring multicast routers.
For example, in the IGMP version 2 (RFC 2236), messages are encapsulated in IP datagrams, comprising at least the type of message and a group address. The group address identifies a multicast channel and is to be understood as the IP multicast group address of the group being reported or left. The IGMP version 2 protocol includes mainly three types of messages in the host-router interaction; 1) a membership query message sent from the router to hosts, 2) a membership report message such as an IGMP “join group” message sent from a host to the router, and 3) an IGMP “leave group” message sent from a host to the router.
The IGMP protocol enables every one of the hosts to request delivery of any multicast channel available from the IGMP router by sending a “join group” message to the router and indicating the channel. The IGMP router registers the host as a member of the IP group and then the host is provided with the channel that has been ordered. It should be noted that in addition to this newly ordered channel, the subscriber unit may receive all the channels ordered by other members of the same IGMP network.
IGMP protocol is a straightforward way of serving multiple subscribers within a passive optical network (PON) that provides multicast services. The customers of PON are usually associated with so-called Optical Network Units ONUs (or Optical Network Terminations ONTs). The OLT (Optical Line Termination) is usually situated in a central office. In the rest of the document, ONT should be understood as either ONT or ONU.
In the case where the OLT comprises an IGMP router, the OLT is in charge of collecting part/all of the IGMP requests from the customers joining a group (as in the conventional IGMP protocol), to figure up which multicast channels are to be delivered to the ONTs. For example, a multicast channel can be ordered by a customer via a remote control unit associated with a Set Top Box (STB) of the customer's TV receiver. The STB cooperates with a corresponding ONT. Upon receiving an IGMP request from one of the customers, OLT transmits the ordered multicast channel downstream, via the PON interface. Each multicast channel is sent by OLT to all members of the IGMP network. The IGMP network may include from 1 to n hosts. Usually n doesn't exceed the maximum number of ONTs where each ONT includes only one IGMP host.
At the PON physical layer, all the information is shared between all ONTs. This fact is due to the physical characteristics of the PON. In order to prevent end-customers to access information not dedicated to them, unicast traffic is usually churned (scrambled) by using a unicast churning function. But such a function is not provided at the PON level for multicast streams. This function, where a key is shared between the OLT and the ONT, can not be used for multicast services where the encryption should be applied per channel or per group of channels. In other words, if a customer served by a specific ONT orders a multicast channel—the ordered channel can be potentially received by all the ONTs of the same PON even if some of these customers are not entitled to see the channel.
A known multicast encryption key exchange method disclosed in a publication US20020150097 comprises selecting/generating a multicast encryption key from among the churning keys belonging to the network devices, and periodically updating the key by delivering to the network devices the difference values between the updated multicast encryption key and the respective churning key of each network device. The process, for the ONT to get the churning key, requests an exchange of several messages between the OLT and ONT and vice-versa. It may become impossible to deliver the encryption/churning information fast enough to the ONT of the customer that just requested a new channel in case the churning key in not known previously by the ONT. In addition, an end multicast customer's ONT may require some service information to access and transmit the multicast channel to the customer (for example, to reserve bandwidth required for the channel's reception and/or transmission to the end equipment though the home network).